The present invention relates generally to the field of optical amplifiers and lasers. More particularly, the present invention relates to a method and apparatus for providing high power pulsed laser sources useful for industrial applications such as trimming, marking, cutting, and welding. Merely by way of example, the invention has been applied to a laser source with real-time tunable characteristics including pulse width, peak power, repetition rate, and pulse shape. However, the present invention has broader applicability and can be applied to other laser sources.
Conventional laser-based material processing has generally used high peak power pulsed lasers, for example, Q-switched Nd:YAG lasers operating at 1064 nm, for marking, engraving, micro-machining, and cutting applications. More recently, laser systems based on fiber gain media have been developed. In some of these fiber-based laser systems, fiber amplifiers are utilized.
Some optical amplifiers and lasers utilizing a fiber gain medium are optically pumped, often by using semiconductor pump lasers. The fiber gain medium is typically made of silica glass doped with rare-earth elements. The choice of the rare-earth elements and the composition of the fiber gain medium depend on the particular application. One such rare-earth element is ytterbium, which is used for optical amplifiers and lasers emitting in the 1020 nm-1100 nm range. Another rare-earth element used in some fiber gain media is erbium, which is used for optical amplifiers and lasers emitting in the 1530 nm-1560 nm range.
The wavelength of the optical pump source used for ytterbium-doped fiber amplifiers and lasers is typically in the wavelength range of 910 nm to 980 nm. The wavelength of the optical pump source used for erbium-doped fiber amplifiers and lasers is typically in a wavelength range centered at about 980 nm or about 1480 nm.
When the laser is operated in a pulse-on-demand mode, the first optical pulse in a series of pulses tends to be more powerful than the following pulses in the series. This situation is sometimes referred to as the first pulse problem and occurs because the energy stored in the laser gain medium, which is depleted significantly after the first pulse, is not fully replenished by the time the next pulse passes through the gain medium. It is possible that if the next pulse arrives after the gain medium is recovered fully, then another pulse similar to the first pulse will be produced. Hence, depending on the pulse repetition rate, which can be varied during the laser operation, the energy in each pulse will generally vary as a function of the state of the gain medium. In laser processing applications, this behaviour is generally undesirable because of the inconsistency in laser pulses and the results achieved during processing operations.
In systems designed to produce a series of high power pulses, the output pulse energy is related to the energy stored in the optical amplifier, which is related to the gain. Generally, to achieve a high energy per pulse, a high gain needs to be created in the optical amplifier, which typically amplifies the first pulse at the expense of the following pulses. If the pulse repetition rate is reduced to enable the gain to recover between pulses, the high gain present in the optical amplifier can create instabilities in the gain medium. Thus, there is a need in the art for high peak power fiber-based amplifiers with repeatable pulse amplification characteristics over a range of pulse repetition frequencies.